ethdev: clarify what is included in generic byte statistics
ethdev: clarify what is included in generic byte statistics

Different hardware gathers statistics differently, so some general
rules need to be established.

Signed-off-by: Viacheslav Galaktionov <3b972c7ef5fe537152d23909a916cdcda27c5dc7@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <4155e7bf8e0185127625dac6d84336299844982f@oktetlabs.ru>
Reviewed-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk